poll: return error on poll() when pollset is empty
poll: return error on poll() when pollset is empty

(cherry picked from commit 222a0d78ca5c272ea109e847d62cf12674dd875f)

mwgoldsmith/libssh,mwgoldsmith/ssh,mwgoldsmith/ssh,mwgoldsmith/ssh,mwgoldsmith/libssh,mwgoldsmith/ssh,mwgoldsmith/libssh,mwgoldsmith/libssh